The overall objective of this proposal is a deeper understanding of long-term nerve-muscle interactions. The research goals are: 1) To analyze early effects of denervation in terms of changes in underlying muscle membrane properties, and, if possible, to use this information to develop a bioassay for the isolation of neurotrophic substances. Denervated muscle will be studied in vitro with physiologic techniques, including measurement of membrane potential and ion content as a function of exposure to solutions of different ionic composition, study of radioactive ion exchanges and fluxes, and measurement of neural acetylcholine release by indirect intracellular recording and direct bioassay. The most sensitive measure of change found in denervated muscle membrane will then be exploited as the basis for an in vitro bioassay of neurotrophic substances. 2) To describe and analyze the process by which denervated muscle accepts and stimulates formation of new synapses. The earliest signs if new synapse formation will be examined in a denervated muscle previously implanted with a foreign motor nerve. This will be studied with intracellular recording, electron microscopy, and assay of choline acetylase activity. By examining membrane properties of the muscle at the time of new synapse formation, it may be possible to determine which denervation changes could and could not act as the stimulus to new synapse formation. 3) To analyze the way in which nerve fibers establish final specific connections with muscle during post-natal development. The results of a quantitative electron microscopic study of a peripheral motor nerve during the post-natal period will be correleated with light microscopic observations on the development of the corresponding muscle and synapses, and with electrophysiological studies of the degree, location, and specificity of branching of peripheral nerve fibers during development.